Receiver selectivity and amplification control



,Aug. 25, 1936. w. VANl B. ROBERTS Filed NOV. 14,A 1932 RECEIVER SELECTIVITY AND AMPLIFIGATION CONTROL (lui SAND 5 INVENTOR- WALTER VAN B. ROBERTS BY #g ATTRNEY- Patented Aug. 25, 1936 PATENT OFFICE RECEIVER SELECTIVITY AND AMPLIFICA- TION CONTROL Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 14. 1932, serial No. 642,564

13 Claims.

The present invention relates to selective high frequency amplifying systems.

It is well known that when dealing with strong signals the radio frequency amplification of a radio receiver should be reduced, and the selectivity should preferably be of a band pass type giving high fidelity. Conversely when weak signals are received the amplification must be increased, and the selectivity should bemade considerably greater. This increase in Vselectivity is desirable in spite of the fact that fidelity of operation is decreased because there is usually so much background noise When receiving Weak signals, that the reduction of this noise by increase of selectivity will, up to a certain point, more than offset the deleterious eifects upon fidelity, so that the best results from the practical point of view are obtained with a much greater value of selectivity than would otherwise be desirable. It is also desirable to have the selecting and amplifying qualities of a receiver independent of the frequency to which it is tuned, in order that all transmitting stations will be received with similar iidelity, and in order that the receiver will not tend toward instability at some adjustments, or lack of amplication at other adjustments.

The present invention has as one of its main objects the provision of means for obtaining both this desirable independence of frequency in the matter of amplification and selectivity, and also a means for adjusting in the same sense the values cf ampliiication and selectivity so that when the single control means is adjusted for reception of strong signals the amplification is maintained uniformly small and the delity uniformly high over the tuning range, and vice versa when the control is adjusted for the reception of weak signals.

Other objects of the invention may be said to comprise the provision of a tuning system whose selectivity is substantially the same at all frequencies to which it may be tuned; the provision of an amplier arrangement whose gain is uniform over the range of frequencies throughout which it may be adjusted to operate; the provision of a system tunable over` a range of frequencies Without variation of either its selectivity or its amplication; the provision of a single means for varying selectivity and amplification simultaneously in the same senses in a system (Cl. Z50-20) the` provision of a combined amplification and vselectivity control in a system having ampliiication and/or selectivity independent of tuning without affecting this independence.

A particular object is to provide a means for practicing tuned radio frequency amplication wherein both amplification and selectivity are independent of tuning adjustments, and a single control means is used for varying values of amplification and selectivity simultaneously and in the same senses.

In the drawing;- Y

Fig. 1 shows an analysis of the invention;

Fig. 2 shows one form of the invention; and

Fig. 3 shows another form of the invention.

In order to demonstrate the necessary conditions for practicing the invention it is desirable to analyze the behavior of the circuit shown in Fig. 1. Taking the voltages around the lefthand mesh I have: Y

di L-l-n-l-q/c-O (1) Taking the voltages around the right hand mesh:

d L2lt1+R(1-q)=0 (2) This is the diierential equation of a Well known and simple type Whose solution is:

In this Equation 5) ,the imaginary part, namely the part under the radical sign of the'coeicient of t determines the natural frequency of oscillation of thel circuit of Fig. 1, while the real part of the coeilicient of t namely:

z' 1 z L-I-wa...l (6) determines the rate of damping of the circuit. This rate of damping is proportional to the width of the resonance curve of the circuit, and, therefore, may be taken as a measure of fidelity.

From the foregoing it will be seen that the fidelity may be maintained independent of the tuning or natural frequency of the system provided C and R are constant while L and r are varied proportionately. There: are, of course, a large number of possibilities in the matt-er of varying the resonant frequency without varying the fidelity, but most of these involve complicated uni-control of a number of thc circuit elements. For this reason, if for no other reason, it would be preferable to Vary only the inductance for tuning purposes. However, there is an additional reason. When this network of Fig. 1 is used as an impedance coupling element in an amplifier system, its impedance remains constant under the conditions of adjustment recommended for constant delity.

I-Ience both amplification and fidelity are independent of tuning so long as C, R, and the ratio of 1 to L are not varied by the tuning adjustment. The impedance of the network at resonance, and hence the amplification, are readily calculated by considering the impedance to be composed of two parallel branches, namely: R and the anti-resonant impedance i rC From this it will be seen that the amplification is decreased as R is made smaller, while the fldelity is made larger as R is made smaller. Thus the adjustment of R determines a value of selectivity rand a value of amplification which are maintained constant over the entire tuning range.

Fig. 2 shows a radio receiving system embodying the invention. To prevent voltage from the plate potential sources B from reaching the grids of the tubes I, 2 blocking condensers K are added to the circuits of Fig. 1, and in order to provide a path for direct current the inductances L are maintained at a high direct current potential. The radio frequency operation of the system, however, is essentially the same as that of Fig. 1. It will be seen that there are two controls provided; one of them, control actuating the variable resistance R of each stage, and thus determining amplification `and fidelity. The other control 3 varies the tuning of each of stages 5 and 6 by varying the value of inductance L in each stage. The control 3 also varies the tuning of a conventionally shown selective input system S, preferably of the band selector type.

In some cases the resistance of the variable inductance L inherently varies with frequency in such a way as to make the resistance Vary substantially proportionately with the inductance. In cases where this does not occur the resistance 1' of the inductive branch of the circuit is varied in uni-control fashion with the inductance in such fashion that the total resistance of the in- Y ductive branch varies directly as the inductance at the frequency to which the system is tuned. The capacities C are in part, and may be wholly, the tube and wiring and other circuit capacities.

It Will now be seen that by maintaining the ratio of r to L constant the tuning of the system no longer varies the selectivity and amplification thereof, while the effect of changing R is to make the selectivity of the entire system approach that of the band selector as R is reduced for the reception of strong signals on the one hand, and to make the overall selectivity sharper than that of the band selector as R is increased for the re- Yment the process is as follows.

ception of Weak signals on the other hand. The remaining elements of the receiver are not described as they are too well known. The stage including tube 2 may serve as the detector.

Fig. 3 shows a particular embodiment of the invention wherein the inductance L is varied by steps. Three steps are shown, but it will be understood that as many as necessary may be used. In designing the tapped inductance ele- Coil L1 is constructed with a suitable amount of resistance T1 to give high amplification and high selectivity at the frequency determined by the inductance L1 and the mean capacity of variable condenser C, which is used to vary the frequency between the steps of the inductance.

Then inductance L2 is designed to have a resistance rz which satisfies the equation where the left side of the equation is evaluated at the frequency range used when both L1+L2 are used, while the right side is evaluated for the frequency range used when only L1 is used. In general the resistance of the nth coil should satisfy the equation the values being determined at the operating frequencies as'explained above. That is, in each case the resistances are the resistances offered by the frequency determined by the inductances in series and the mean value of capacity C.

The variable resistance R has exactly the same effect as in Fig. 2. The fact that a variable condenser is used violates the conditions for absolute constancy of amplification and selectivity, but if the taps 1, 9 are chosen to give reason.- ably small changes in inductance (such as, for example, each successive tap multiplying the inductance by two) a satisfactory approximation to constancy will result. The arrangement of Fig. 3 is particularly suitable for multi-Wave receivers, as the first inductance may be made small enough to tune the system to the shortest wave lengths while successive inductances may be switched in by arm l so as to bring the wave length up to any desired value.

It will be understood that in order to avoid losses when working at high frequencies, each section of the inductance should be maintained entirely uncoupled by shielding or otherwise, from all of the other sections. Further, the basic principle shown in Fig. l and its analysis may be embodied in many modifications of the circuits shown in Figs. 2 and 3.

What I claim is: K

l. In a radio frequency amplifier provided with a resonant input circuit coupled to a signal energy source, means for tuning the circuit over a desired signal frequency range, said tuning means including reactive and resistive impedances whose constants are related throughout the tuning adjustment of the circuit to maintain the selectivity and amplification independent of the circuit frequency, additional means in the circuit for adjusting at will the values of said amplification and selectivity in the same sense, said additional means being physically independent of said tuning means and being constructed and arranged to have no electrical effect upon the resonant frequency of the circuit at any adjustment of the tuning means.

2. In a radio frequency amplifier provided with a resonant input circuit coupled to a source of signal energy, means for tuning the circuit through a desired frequency range in such a manner that the selectivity and amplification are independent of the tuning operation, and Vadditional means constructed to have no electrical effect on the resonant frequency of said circuit for adjusting in the same sense the values of amplification and selectivity, said additional means being a normally variable resistor;

3. In a radio frequency amplifier provided with a resonant input circuit coupled to a source of signal energy, means for tuning the circuit through a desired frequency range in such a manner that the selectivity and amplification are independent of the tuning operation, and additional means in said circuit for adjusting in the same sense the values of amplification and selectivity, said additional means being constructed to have no electrical effect'on the resonant` frequency of the circuit, and a band selector unit coupledto the circuit, including tuning means adjustable with said first tuning means, for preventing the overall selectivity of the amplifier from getting too broad.

4. In a radio receiver, a plurality of lugh frequency amplifier stages, means, comprising a variable inductive reactance in each stage, for tuning the stages to a desired signal frequency of a predetermined signal frequency range, the relation between inductive reactance and resistance in each of said stages being such that the selectivity and amplification of the receiver are independent of the tuning operation, and an addltional variable impedance means in each of said stages constructed to have no electrical effect on the resonant frequency thereof for varying at will the selectivity and amplification simultaneously and in the same sense.

5. In a radio receiver, a plurality of high frequency amplifier stages, means, comprising a variable inductive reactance in each stage, for tuning the stages to a desired signal frequency of a predetermined signal frequency range, the relation between inductive reactance and resistance in each of said stages being such that the selectivity and amplification of the receiver are independent of the tuning operation, and an addi` tional variable impedance means in each of said stages constructed to have no electrical effect on the resonant frequency thereof for varying at will the selectivity and amplification simultaneously and in the same sense, and a band selector unit to prevent the overall selectivity of the receiver from getting too broad, and means for tuning the band selector simultaneously with said stages.

6. In a radio receiver, a radio frequency amplifier provided with a resonant input circuit and an output load circuit, a source of signal energy coupled to said input circuit, a capacitative means for tuning the circuit through a desired frequency range, an adjustable inductive reactance in said circuit which is related to said tuning means in such a manner that the selectivity and amplification are independent of the tuning operation, and an additional means, including an adjustable resistor having no electrical effect on the resonant frequency of the input circuit, in said circuit for adjusting in opposite senses the values of amplification and fidelity.

7. In a radio receiver, a radio frequency amplifier provided with a resonant input circuit and an output load circuit, a source of signal energy coupled to said input circuit, a capacitative means for tuning the circuit through a' desired frequency range, an adjustable inductive reactance in said circuit which is related to said tuning means in such `a manner that the selectivity and amplification are independent of the tuning operation, and an additional means, including an adjustable resistor having-no electrical effect on the resonant frequency of the input circuit, in said circuit for adjusting in opposite senses the values of amplification and fidelity so that when the tuning means is adjusted for reception of strong signals the amplification is maintained uniformly small and the fidelty uniformly high over'the tuning range, and vice versa when the tuning control is adjusted for the reception of weak signals. Y,

8. A transmission network adapted to couple a pair of electron discharge tubes, said network including a reactance connected across the output electrodes of one of saidtubes, a variable tuning reactance of opposite electrical sign in shunt with said first reactance, the said reactances, and resistance in series with the variable reactance, being so chosen that the amplification and selectivity of the network are independent of the tuning operation, and a variable resistor connected across the input electrodes of the second tube for varying the fidelity of the network, adjustment of the variable resistor having no electrical effect on the resonant frequency of the network, the amplification being inherently varied 1n an opposite sense.

9. In a receiver a pair of amplifier tubes', a network coupling the output electrodes of a first of said tubes to the input electrodes of the following tube, said network including an inductance coil provided with a plurality of spaced tapped points, a variable condenser connected across the output electrodes' of the preceding tube, and means for connecting at will one side of the variable condenser to any one of said coil taps, and a variable selectivity and amplification control device electrically unaffected by adjustment of the variable condenser connected across the input electrodes of the succeeding tube, the resonant frequency of the coupling network being unaffected by the operation of said amplification and selectivity control device.

10. In a radio receiver, a high frequency amplifier provided with an input circuit and an output circuit, a source of signal energy, a band selector unit coupled between the signal energy source and the said input circuit, said input circuit including a resonant network comprising a fixed reactance and a variable reactance of an opposite electrical sign, the resistance of said resonant network being so chosen that the amplification and selectivity of the network are independent of tuning, a common means for varying the tuning of the said network with the tuning of the band selector, and an adjustable impedance in said input circuit, having no electrical effect on the resonant frequency of the latter, for adjusting in opposite senses the values of amplification and fidelity of said receiver so that when the tuning control means is adjusted for reception of strong' signals the amplification isv pling the output electrodes of the latter to the 75 input electrodes of the second tube, said network comprising a condenser across the said output electrodes,` a path including a variable inductor connected across'the condenser, and a variable resistor connected between the input electrodes of the second tube, means for adjusting the magnitude` of said inductor while maintaining constant the ratio of resistance of said path to inductance thereof to tune said network over a desired frequency range, and additional means for adjusting the value of said resistor to Vary the selectivity of 'said network at any desired adjustment of said tuning adjusting means.

12. In a radio receiver, at least two cascaded tubes, a source of signal energy coupled to the input electrodes of the first tube, a network coupling the output electrodes of the latter to the input electrodes of the second tube, said network comprising a condenser across the said output electrodes, a path including a variable inductor and a variable resistance connected across the condenser,` and a variable resistor connected between the input electrodes of the second tube, means for adjusting the magnitude of said inductor while maintaining constant the ratio of resistance of said path to inductance thereof to input electrodes of the second tube, said network 1,0

comprising a condenser across the said output electrodes, a path including a variable inductor connected across the condenser, and a variable resistor connected between the input electrodes of the second tube, means for adjusting the magnitude of said inductor while maintaining constant the ratio of resistance of said path to inductance thereof to tune said network over a desired frequency range, and additional means for adjusting the Value of said resistor to vary the 219g;

selectivity of said network at any desired adjustment of said tuning adjusting means, and a band selector unit coupled between said source and rst tube.

WALTER VAN B. ROBERTS. 

